1. Field of the Invention
The present invention relates to the field of baluns. Such transformers are intended to convert a common-mode signal into a differential-mode signal and conversely, and to avoid disturbances introduced by a ground plane of the circuit, a parasitic coupler, etc.
Baluns generally equip radio transceiver chains. They are in charge of converting common-mode signals received by an antenna into differential signals exploitable by processing circuits and, conversely, of converting differential signals to be transmitted into a common-mode signal for the antenna.
2. Discussion of the Related Art
A balun comprises a common-mode access, generally on the side of a communication antenna, and two differential-mode accesses intended to be connected to circuits for processing received signals or signals to be transmitted. Electrically, the balun is equivalent to an inductive assembly comprising coupled inductances. Typically, an inductance (or two inductances in series) connect the common-mode access to a terminal which may be floating or grounded by a capacitive element. An inductance with a midpoint or two inductances in series connect the differential-mode accesses, the midpoint of this series association being generally grounded.
Baluns are divided in three categories. A first category relates to so-called distributed baluns, formed of transmission lines coupled to one another. A second category relates to so-called active baluns, formed of transistors. A third category relates to so-called lumped baluns, formed of inductive and possibly discrete capacitive components. The present invention relates to distributed baluns of the first category.
The features which are desired to be respected are low insertion losses, a good impedance matching, and minimum phase and amplitude imbalances between the differential paths.
When the balun is associated with a transmit chain, an oscillator providing a frequency to a modulator of the data to be transmitted is generally placed upstream. The oscillator (typically, a voltage-controlled oscillator—VCO) generally delivers a signal at a frequency which is a multiple (generally, double or half) of the carrier frequency (of the useful signal).
The circuits connected downstream of the modulator are generally sized according to the central frequency of the useful band. Accordingly, their impedance at another frequency is generally not optimized.
In case of a variation of the impedance seen by the oscillator, the oscillator is likely to undergo a frequency drift. This phenomenon is generally called “pulling”. Now, in a transmit chain, a variation of the oscillator frequency generates a variation of the carrier frequency.
Theoretically, the balun should exhibit a given impedance at the frequency of the useful signal and another given impedance at the oscillator frequency. The two criteria are often incompatible with usual balun structures.
US-A-2008/303606 discloses a wide-band balun in which the common mode winding is in series with a capacitor connected between said winding and the terminal on which the signal is applied, the other terminal of the common mode winding being directly grounded.